Normal individuals experience several distinct sleep states. One important sleep state is characterized by Rapid Eye Movement, known as REM sleep, small muscle twitches, changes in autonomic activity and the absence of other body movements. The other sleep state, known as Non-REM (NREM) sleep is subdivided into four stages, wherein the first stage is the most shallow, i.e., the least restful or refreshing, and the fourth stage is the deepest.
Monitoring an individual's sleep is very important for diagnosing sleep disorders. It also is useful in various fields, such as therapy, diagnosing and following response to treatment of depression and narcolepsy in which REM latency is significantly reduced, or in research.
For diagnosis, the patient's sleep stages should be monitored to determine the pattern and duration of various sleep stages. Sleep is qualitatively and quantitatively evaluated by measuring electrical signals produced by brain and muscle activity, using electrophysiological technique and electronic instruments.
A widely used technique for this purpose, involves a simultaneous and continuous measuring of electroencephaographic (EEG) data. EEG data are signals derived primarily from the cortex of the brain and also are referred to as electrocortigram (ECoG). At the same time an electromyogram (EMG) signal which monitors muscle activity, generally from one of the muscles of the lower jaw is measured, together with left eye and right eye electro-oculogram (EOG) signals produced by eye movements. These EEG, EMG and EOG signals are conventionally recorded on a multichannel physiological recorder.
Photographic techniques also have been used to evaluate sleep state. According to Hobson et al.(Science 201,1978, p.1251-5), the mobility of sleeping subjects is measured photographically and the predicted transitions between NREM and REM are done on the premises that major body posture shifts occur immediately preceeding and following REM sleep.
According to U.S. Pat. No. 4,784,162, an apparatus is described for monitoring sleep disorders using a plurality of sensors adapted for attachment to a patient. The sensors generate analog signals which are subsequently converted to binary data to be transmitted by low-power radio. According to U.S. Pat. No. 4,836,219, a method and device are described for reporting an individual's sleep state using electronic filters applied to analog signals representative of eye movements and head movements from detectors attached to a headgear.
Among the main disadvantages of the above methods, there can be mentioned the following:
A high rate of false positive REM detections, by providing a signal indicating the occurrence of REM sleep, when no REM sleep actually is involved. The reason is that the eye movement detector also indicates pick gross body movements which are not part of the typical changes in REM sleep.
The robust construction of the various components as well as the costly equipment and skilled labour required for the interpretation of the results obtained.
In view of the difficulties with existing sleep evaluation techniques, there are many cases, where sleep disorders are not tested with the above known techniques and are treated with sedative-hypnotic drugs without a prior detailed sleep evaluation.
It is an object of the present invention to provide a simple and inexpensive system to determine a non-invasive sleep state. It is another object of the present invention to provide a simple system to determine an accurate non-invasive sleep state. It is yet another object of the present invention, to provide a system for determining a non-invasive sleep state, which does not require skilled personnel.